2 Genetic Code and RNA

7/30/2019 2 Genetic Code and RNA

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5.6 The sequence ofbases in the DNA

determines thestructure of proteins,

including enzymes.


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Specification

The genetic code as base triplets in mRNA

which code for specific amino acids.

The genetic code is universal, non-overlapping

and degenerate. The structure of molecules of messenger RNA

(mRNA) and transfer RNA (tRNA).

Candidates should be able to compare thestructure and composition of DNA, mRNA and

tRNA.


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The Genetic Code

A sequence of three bases, called a triplet, codes

for a specific amino acid.

Amino acid 1 Amino acid 2 Amino acid 3 Amino acid 4


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The order in which

the triplets appeardetermines the

sequence in which

the amino acids are

combined to make

the polypeptide

chain


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1st

Base

2nd

Base

3rd

Base

T C A G

T

Phenylalanine Serine Tyrosine Cysteine T

Phenylalanine Serine Tyrosine Cysteine C

Leucine Serine Stop Stop A

Leucine Serine Stop Tryptophan G

C

Leucine Proline Histidine Arginine T

Leucine Proline Histidine Arginine C

Leucine Proline Glutamine Arginine A

Leucine Proline Glutamine Arginine G

A

Isoleucine Threonine Asparagine Serine T

Isoleucine Threonine Asparagine Serine C

Isoleucine Threonine Lysine Arginine A

Methionine

(Start)1

Threonine Lysine Arginine G

G

Valine Alanine Aspartate Glycine T

Valine Alanine Aspartate Glycine C

Valine Alanine Glutamate Glycine A

Valine Alanine Glutamate Glycine G
http://www.soc-bdr.org/rds/authors/unit_tables_conversions_and_genetic_dictionaries/e5202/index_en.htmlhttp://www.soc-bdr.org/rds/authors/unit_tables_conversions_and_genetic_dictionaries/e5202/index_en.html


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The code is

Universal all living things share the same

four organic bases

Degenerate one amino acid can be

coded for by more than one triplet

Non-overlapping the triplets are all

sequential and it matters where you start


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Guess the

nucleic

acid?


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Compare:

RNA

Uracil pairs with Adenine

Ribose sugar

Usually single stranded

Not arranged in a helix

Relatively short

DNA

Thymine pairs with adenine

Deoxyribose sugar

Double stranded

Arranged in a double helix

Much longer


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Function of RNA

It transmits genetic information stored in DNA:

Transcription: it expresses the genetic

information as mRNA

Translation: it transports amino acids to the

ribosome in protein synthesis (tRNA)


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Messenger RNA

mRNA

a template for

protein

synthesis made in the

nucleus when

free RNA

nucleotides

match up with

DNA template


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Transfer RNA

tRNA

The RNA molecule is

formed into a

secondary structurelike a cloverleaf

Transfers amino acids

to the ribosome

during protein

synthesis.


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tRNA molecule

The amino acidcollected by the

tRNA molecule is

determined by the

arrangement ofbases at the

anticodonAnticodon

Amino acid attaches here


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Base sequences

DNA triplet mRNA codon tRNA - anticodon(phenylalanine)


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Ribosomal RNA

rRNA

Combines with protein to

form ribosomes


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Describe how proteins are

synthesised using the DNA code

enzymes break weak hydrogen bonds betweenthe bases

one strand of DNA acts as a template

to form a molecule of mRNA by the process of complimentary base pairing

(C=G, T=A, A=U, G=C)

free nucleotides are found in the nucleoplasm

mRNA molecule leaves nucleus via nuclearpores

travels to ribosomes in cytoplasm


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ribosome attaches to mRNA at a start code

a tRNA molecule with a complimentary

anticodon will bind to the appropriate codon onthe mRNA

this tRNA molecule will bring a particular aminoacid to the ribosome

sequence of amino acids is determined by thesequence of bases in DNA chain

peptide bonds form between amino acids

a polypeptide chain is formed this is transported to the endoplasmic reticulum(ER) to be folded into a protein

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2 Genetic Code and RNA